Recently a communication conference (talk session) unit equipped with a speaker and a microphone becomes widespread. The communication conference unit dials in to the conference server via the telephone line network. When the communication conference units set up at plural points dial in to the conference server respectively, users can have a voice talk with other users.
Meanwhile, recently VoIP (Voice over Internet Protocol) that holds a voice talk on the Internet Protocol (referred to as “IP” hereinafter) is spreading. When the user holds a voice talk on IP, the communication conference unit accesses to an IP address of the IP conference server (referred to as “MCU (Multipoint Control Unit)” hereinafter) and transmits/receives voice information while using Protocols such as TCP (Transmission Control Protocol), SIP (Session Initiation Protocol), and the like.
The telephone line network is diffused conventionally, and VoIP is becoming rapidly common in recent years. Therefore, the user who has only the connecting unit to the conference server via the conventional telephone line network and the user who has only the connecting unit to MCU via IP exist. The user who has only the connecting unit to the conference server via the conventional telephone line network cannot access to MCU via IP, while the user who has only the connecting unit to MCU via IP cannot access to the conference server via the telephone line network.
As a result, the architecture that can link the user who has only the connecting unit to the conference server via the conventional telephone line network and the user who has only the connecting unit to MCU via IP mutually has been proposed (see Patent Literature 1, for example).    Patent Literature 1: WO02063840 (JP-T-2004-527156)